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Molecular line and continuum studies of the early stages of star formation

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UNIVERSITY OF HELSINKI REPORT SERIES IN ASTRONOMY No. 1 Molecular line and continuum studies of the early stages of star formation Oskari Miettinen Academic dissertation Department of Physics Faculty of
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UNIVERSITY OF HELSINKI REPORT SERIES IN ASTRONOMY No. 1 Molecular line and continuum studies of the early stages of star formation Oskari Miettinen Academic dissertation Department of Physics Faculty of Science University of Helsinki Helsinki, Finland To be presented, with the permission of the Faculty of Science of the University of Helsinki, for public criticism in Auditorium XV of the University Main Building on 19 November 2010, at 12 o clock noon. Helsinki 2010 Cover picture: The IRAM 30-m telescope on Pico Veleta in the Spanish Sierra Nevada. Photo taken by O. Miettinen. ISSN (printed version) ISBN (printed version) Helsinki 2010 Helsinki University Printing House (Yliopistopaino) ISSN (pdf version) ISBN (pdf version) ISSN-L Helsinki 2010 Electronic University of Helsinki (Helsingin yliopiston verkkojulkaisut) Oskari Miettinen: Molecular line and continuum studies of the early stages of star formation, University of Helsinki, 2010, 98 p.+appendices, University of Helsinki Report Series in Astronomy, No. 1, ISSN (printed version), ISBN (printed version), ISSN (pdf version), ISBN (pdf version), ISSN-L Classification (INSPEC): A9580D, A9580E, A9580G, A9710B, A9720D, A9840B, A9840C, A9840J, A9840K, A9840L Keywords: interstellar medium, molecular clouds, clumps, cores, star formation, molecular spectral lines, dust continuum, radio continuum Abstract New stars form in dense interstellar clouds of gas and dust called molecular clouds. The actual sites where the process of star formation takes place are the dense clumps and cores deeply embedded in molecular clouds. The details of the star formation process are complex and not completely understood. Thus, determining the physical and chemical properties of molecular cloud cores is necessary for a better understanding of how stars are formed. Some of the main features of the origin of low-mass stars, like the Sun, are already relatively well-known, though many details of the process are still under debate. The mechanism through which high-mass stars form, on the other hand, is poorly understood. Although it is likely that the formation of high-mass stars shares many properties similar to those of low-mass stars, the very first steps of the evolutionary sequence are unclear. Observational studies of star formation are carried out particularly at infrared, submillimetre, millimetre, and radio wavelengths. Much of our knowledge about the early stages of star formation in our Milky Way galaxy is obtained through molecular spectral line and dust continuum observations. The continuum emission of cold dust is one of the best tracers of the column density of molecular hydrogen, the main constituent of molecular clouds. Consequently, dust continuum observations provide a powerful tool to map large portions across molecular clouds, and to identify the dense star-forming sites within them. Molecular line observations, on the other hand, provide information on the gas kinematics and temperature. Together, these two observational tools provide an efficient way to study the dense interstellar gas and the associated dust that form new stars. The properties of highly obscured young stars can be further examined through radio continuum observations at centimetre wavelengths. For example, radio continuum emission carries useful information on conditions in the protostar+disk interaction region where protostellar jets are launched. In this PhD thesis, we study the physical and chemical properties of dense clumps and cores in both low- and high-mass star-forming regions. The sources are mainly studied in a statistical sense, but also in more detail. In this way, we are able to examine the general characteristics of the early stages of star formation, cloud properties on large scales (such as fragmentation), and some of the initial conditions of the collapse process that leads to the formation of a star. The studies presented in this thesis are mainly based on molecular line and dust continuum observations. These are combined with i archival observations at infrared wavelengths in order to study the protostellar content of the cloud cores. In addition, centimetre radio continuum emission from young stellar objects (YSOs; i.e., protostars and pre-main sequence stars) is studied in this thesis to determine their evolutionary stages. The main results of this thesis are as follows: i) filamentary and sheet-like molecular cloud structures, such as infrared dark clouds (IRDCs), are likely to be caused by supersonic turbulence but their fragmentation at the scale of cores could be due to gravothermal instability; ii) the core evolution in the Orion B9 star-forming region appears to be dynamic and the role played by slow ambipolar diffusion in the formation and collapse of the cores may not be significant; iii) the study of the R CrA star-forming region suggests that the centimetre radio emission properties of a YSO are likely to change with its evolutionary stage; iv) the IRDC G contains candidate high-mass starless cores which may represent the very first steps of high-mass star and star cluster formation; v) SiO outflow signatures are seen in several high-mass star-forming regions which suggest that high-mass stars form in a similar way as their low-mass counterparts, i.e., via disk accretion. The results presented in this thesis provide constraints on the initial conditions and early stages of both low- and high-mass star formation. In particular, this thesis presents several observational results on the early stages of clustered star formation, which is the dominant mode of star formation in our Galaxy. ii Acknowledgements Most of the work carried out for this PhD thesis has been done at the Observatory of the University of Helsinki. The historical observatory building offered a great environment to work on astronomy. The present thesis was completed at the Department of Physics to which the Department of Astronomy merged with at the beginning of First of all, I am deeply grateful to my thesis supervisor, Docent Dr. Jorma Harju. His continuous help, advice, and encouragement (to mention just a few things) through all these years have been indispensable. I thank Docent Dr. Lauri Haikala for acting as a second supervisor, and in particular for his guidance in the dust continuum studies. I wish to express my sincere gratitude to Professor Dr. Kalevi Mattila for giving me the chance to work in the Interstellar Medium and Star Formation-research group at the observatory already during my second year of study. In addition to the above named persons, I have received help in one way or another from many other people, and they are all greatly acknowledged (forgive me not mentioning you all here). It has been my pleasure to have collaborated with all of you. Also, thank you to everybody who read and commented different parts of this thesis. I am indebted to the pre-examiners Professor Dr. René Liseau (Onsala) and Dr. Friedrich Wyrowski (Max-Planck-Institut für Radioastronomie (MPIfR), Bonn), for taking some of their valuable time to review this thesis. For the financial support during the thesis work, I am grateful to the Finnish Graduate School in Astronomy and Space Physics, the Research Foundation of the University of Helsinki, and the Academy of Finland. Moreover, the Magnus Ehrnrooth Foundation is acknowledged for providing travel support. I would like to thank the staff at the IRAM 30-m telescope for their hospitality and help during the observations presented in Paper I of this thesis. Doctor Alex Kraus and the operators of the Effelsberg 100-m telescope are thanked for their help during the observations presented in Paper II. I also should sincerely thank the staff at the APEX telescope in Chile; many of the observations presented in this thesis are carried out with APEX in service mode. I would like to thank Dr. Martin Hennemann, who is currently at CEA Saclay (France), and Dr. Hendrik Linz at the Max-Planck-Institut für Astronomie (MPIA) for their collaboration with several proposals concerning the studies of infrared dark clouds, and for their kind hospitality during my visit to MPIA. Doctor Jouni Kainulainen, the former member of the ISM/SF-group, and who is currently at the MPIA, is thanked for several collaborations (e.g., Paper I and solving all sorts of never ending computer problems). Thank you, Laura, for your love, care and kind-heartedness. Finally, very special thanks go to my mother for her outstanding support. Oskari Miettinen Helsinki, September 2010 iii List of publications This thesis consists of an introductory review part, followed by five research publications: Paper I: Miettinen, O., Harju, J., Haikala, L. K., Kainulainen, J., and Johansson, L. E. B., Prestellar and protostellar cores in Orion B9, 2009, A&A, 500, 845 Paper II: Miettinen, O., Harju, J., Haikala, L. K., and Juvela, M., Physical properties of dense cores in Orion B9, 2010, A&A, in press Paper III: Miettinen, O., Kontinen, S., Harju, J., and Higdon, J. L., Radio continuum imaging of the R Coronae Austrinae star-forming region with the ATCA, 2008, A&A, 486, 799 Paper IV: Miettinen, O., and Harju, J., LABOCA mapping of the infrared dark cloud MSXDC G , 2010, A&A, in press Paper V: Miettinen, O., Harju, J., Haikala, L. K., and Pomrén, C., SiO and CH 3 CCH abundances and dust emission in high-mass star-forming cores, 2006, A&A, 460, 721 These papers will be referred to in the text by their Roman numerals, and are summarised in Chapter 7, where also author s contribution are described. The articles are reprinted with kind permission of Astronomy and Astrophysics. iv List of abbreviations AD ALMA APEX ATCA BE CMF CTTS FIR GMC HC HH HMC HPBW HMPO HMSC IMF IR IRAM IRAS IRDC ISM ISRF KL K-S LABOCA LTE MHD MIR MSX NIR PI PMS SED SEST SFE SFR SIMBA SN UC UV WTTS YSO ZAMS Ambipolar diffusion Atacama Large Millimetre/submillimetre Array Atacama Pathfinder Experiment Australia Telescope Compact Array Bonnor-Ebert (sphere) Core mass function Classical T Tauri star Far infrared Giant molecular cloud Hypercompact (HII region) Herbig-Haro (object) Hot molecular core Half-power beam width High-mass protostellar object High-mass starless core Initial mass function Infrared Institut de Radioastronomie Millimétrique Infrared Astronomical Satellite Infrared dark cloud Interstellar medium Interstellar radiation field Kleinmann-Low (nebula) Kolmogorov-Smirnov (test) Large APEX Bolometer Camera Local thermodynamic equilibrium Magnetohydrodynamic Mid-infrared Midcourse Space Experiment Near infrared Principal investigator Pre-main sequence (star) Spectral energy distribution Swedish-ESO Submillimetre Telescope Star formation efficiency Star formation rate SEST Imaging Bolometer Array Supernova (plural SNe) Ultra-compact (HII region) Ultraviolet Weak-line T Tauri star Young stellar object Zero-age main sequence v Contents 1 Introduction Background Purpose and scope of this thesis Structure of the thesis Observational techniques and tools 4 3 Basic equations Molecular column density calculation H 2 column density from (sub)millimetre dust continuum emission Mass determination from dust continuum emission Spectral index of thermal radio continuum emission Low-mass star formation Low-mass starless/prestellar cores Physical properties of prestellar cores Gas dynamics, kinematics, and thermodynamics The role of magnetic field in the core dynamics Chemistry of prestellar cores Cosmic-ray ionisation and the ionisation degree Molecular freeze-out Deuterium fractionation Protostellar cores, protostars and young stellar objects Spectral energy distribution of YSOs Class 0 sources Class I sources Class II sources Class III sources Jets and outflows associated with protostellar cores Shock chemistry in molecular outflows Radio continuum emission from YSOs Thermal radio emission Non-thermal radio emission Connection between the radio continuum emission of a YSO and its evolutionary stage Lifetime of the prestellar phase of core evolution vi Contents 4.8 Ambipolar diffusion and the standard model of low-mass star formation Observational support for the AD theory Observational evidence against the AD theory High-mass star formation Introduction Infrared dark clouds Substructures within IRDCs High-mass protostellar objects Hot cores Hyper- and ultra-compact HII regions Disks and outflows in high-mass star-forming regions Alternative formation mechanisms for high-mass stars Competitive accretion Coalescence model Issues on turbulence, molecular cloud fragmentation, and control of star formation Turbulence and molecular cloud fragmentation the origin of clumps and cores within molecular clouds Fragmentation of IRDCs Clump and core mass distributions Spatial distribution of clumps and cores within molecular clouds Core/star formation efficiency Turbulence versus ambipolar diffusion driven star formation Summary of the publications Paper I Paper II Paper III Paper IV Paper V Concluding remarks 81 Bibliography 83 vii Chapter 1 Introduction 1.1 Background Star formation has an essential role in the universe as it plays a key role in determining the structure and evolution of galaxies. It is thus not surprising that star formation studies have become an integral part of modern astrophysics. Stars form in interstellar molecular clouds that consist of gas (mostly molecular hydrogen, H 2 ) with a small fraction of dust. The entire molecular gas mass of the Galaxy is 10 9 M. Most of the molecular material is in the form of giant molecular clouds (GMCs), which are the largest structures within our Galaxy and the primary sites of star formation. They have masses of M, sizes from 20 to 100 pc, gas kinetic temperatures of K, and average H 2 densities of n(h 2 ) cm 3 (e.g., Blitz 1993). More precisely, it is the dense clumps and cores within molecular clouds where the gravitational collapse and the actual star formation take place; the terms clump and core are often used to refer to objects with masses, sizes, and mean densities of M, pc, cm 3, and 1 10 M, 0.1 pc, cm 3, respectively (e.g., Bergin & Tafalla 2007). It is thus important to study the properties and dynamical evolution of these objects. On the other hand, the origin of dense cores is not yet fully understood. Shock compression by turbulent flows within molecular clouds is considered to be a likely mechanism of core formation. The salient feature of star formation in our Galaxy is that most stars form in groups and clusters which contain tens to hundreds of objects, whereas isolated star formation is rare (e.g., Lada & Lada 2003). The most detailed observational studies and theoretical models deal with low-mass star formation in isolated dense cores (e.g., Shu et al. 1987, 2004). It is useful to extend the investigations to the regions where stars form in clustered mode. The star formation process is governed by the interplay between gravity, gas dynamics, turbulence, magnetic fields, and both electromagnetic and cosmic-ray radiation. The details of the physical processes, however, remain an open question. For example, it is still a matter of debate what is the relative importance of turbulence and magnetic fields (McKee & Ostriker 2007). In addition to the processes mentioned above it has also become clear that interstellar chemistry is of utmost importance to star formation studies. Chemistry plays a role in the ionisation degree of the gas, and controls, e.g., the cooling of the gas. Thus, chemistry affects the dynamics of the star-forming core. The main features of the process of low-mass star formation are already relatively well 1 Chapter 1 Introduction understood. For example, low-mass dense cores can be distinguished into several stages which are likely to represent an evolutionary sequence, i.e., starless cores, prestellar cores, Class 0 I protostellar cores and, furtheron, Class II and III pre-main sequence (PMS) stars. In contrast, the formation of high-mass stars, which are more important for the evolution of galaxies, is still poorly understood. There are several reasons for this. Especially, high-mass stars and their formation sites are rare, and high-mass stars form in highly clustered regions making it diffucult to determine which processes are at play. Nevertheless, in recent years the knowledge of high-mass star formation has increased considerably. This is, in part, due to the discovery of the so-called infrared dark clouds (IRDCs) and the clumps and cores within them, some of which are likely to represent the earliest stages of high-mass star and star cluster formation. Particularly, IRDCs may contain high-mass analogues of cold low-mass prestellar cores. Among the most useful observational tools to study the earliest stages of star formation are molecular spectral lines and dust continuum emission. Molecular lines provide information on the gas temperature, kinematics (e.g., turbulence and infall), and molecular abundances. Continuum emission of dust can be used to determine the basic properties of star-forming structures, such as mass, size, and column density of H 2. Moreover, when studying objects that are already in the protostellar or PMS stage, radio continuum emission, such as thermal free-free emission from ionised gas, can be used to study e.g, the properties of the circumstellar environment and associated protostellar winds and jets. 1.2 Purpose and scope of this thesis In this thesis, both low- and high-mass star-forming regions are investigated by means of molecular lines, dust continuum, and radio continuum observations. Studies of the physical and chemical properties of star-forming cores presented in this thesis provide information on the initial conditions and early stages of star formation. This is the main purpose of the thesis. Because the studied regions represent clustered regions of star formation, the results of our studies provide useful information on the dominant star formation mode in the Galaxy. Most of the studies presented in this thesis are survey-like and address statistical properties. For some individual objects also more detailed studies of the physical and chemical properties are presented. The latter studies are needed to shed light on, e.g., the protostellar collapse. In particular, because the thesis include studies on both the low- and high-mass star formation, the results obtained can help to answer the question whether the formation of low- and high-mass stars can proceed in a similar manner. 1.3 Structure of the thesis The thesis consists of an introductory review part and five original publications. Three of the papers are published in the international peer-review journal Astronomy and Astrophysics (A&A), and two of them will appear in the same journal (in press). 2 Chapter 1 Introduction The introductory part of the thesis is organised as follows. In Chapter 2, the observational techniques and tools are briefly described, including descriptions of the telescopes and instruments used in the studies. In Chapter 3, the most relevant equations for this thesis are derived. In Chapter 4, an overview of low-mass star formation is given, emphasising the topics which are relevant for the thesis (e.g., prestellar cores). Chapter 5 is dedicated to high-mass star formation. Large-scale properties (or macrophysics ) of star formation in Galactic molecular clouds are discussed in Chapter 6, the emphasis being in the interstellar turbulence and how it is related to the origin of dense cores and their mass distribution. Summaries of Papers I V are given in Chapter 7, including a description of the authors contribution to the papers. Finally, concluding remarks are presented in Chapter 8. The original publications are included at the end of the thesis. 3 Chapter 2 Observational techniques and tools The star formation process is obscured by large amounts of gas and dust in the dense interiors of molecular clouds. The bulk of the associated matter is v
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